Publish whitelist

per

https://github.com/github/pages-gem/issues/604#issuecomment-446997438

it appears i have a misunderstanding of what this is:

http://pages.github.com/versions

my understanding is that page is a list of all plugins that are whitelisted by GitHub pages.

if thats not the case - can we get a public list of those

or are we expected to just look here:

https://github.com/github/pages-gem/blob/fb5c3f22a5789aee30c942e30962a69a8ac75e3a/lib/github-pages/plugins.rb#L22-L41